Field
This invention relates generally to active solid-state devices, and more specifically to a dynamic bias circuit for a fast Wi-Fi switch.
Related Art
Many fast switches for Wi-Fi access and similar applications drive the switches using locally generated biases and low impedance drivers. An example of such a switch 100 is shown in FIG. 1. Disadvantageously, a radio frequency (RF) signal at node RF100 is AC coupled to an output load (represented by R100) using capacitor C100 which increases a size of the switch 100. With the switch 100 of FIG. 1, a common node N100 is AC coupled via capacitor C100 to an output RF100, and is DC coupled via resistor R101 to Vdd. A negative bias voltage Vgs of the switch 100 can, therefore, be driven from −Vdd to 0V.
The switch 100 routes an RF signal from one of two channels RF101, RF102 to RF100 depending upon the bias voltage at the gates of the two FETs Q101, Q102. The bias voltages at the gates of Q101 and Q102 are set by the values of the control inputs P101, P102 of amplifiers digital logic gate U101, U102. An RF coupling capacitor C101, C102 is required for each channel RF101, RF102 and the common output RF100 to maintain this bias scheme. Because a swing of Vgs is from only −Vdd to ground, a power level of the output is limited.
An integrated solution of a fast switch 200 for Wi-Fi access and similar applications, which uses DC coupling at all RF ports is shown in FIG. 2. The switch 200 provides multiple RF ports (N), with each RF port controlled by a similar bias block. The integrated solution 200 shown in FIG. 2 includes an input decoder 202, a driver bias generator block 204, N level shifters/drivers 206a, 206b, . . . , 206N (referenced collectively and generally as level shifter/driver 206) and a negative voltage generator 208 (e.g., a charge pump). The integrated solution 200 shown in FIG. 2 allows DC coupling at all output ports P200a, P200b, . . . , P200N, i.e., coupling capacitors are not used. Control inputs P201 and P202 determine which RF signal RF201a, RF201b, . . . , RF201N to deliver to the RF Out node RF200. The input decoder 202 determines which of the level shifter/drivers 206a . . . 206N to activate to switch the corresponding RF signal RF201a . . . RF201N to be delivered to the RF Out node RF200 according to the values presented at the control inputs P201, P202. The voltage at the control inputs P201, P202 typically range from 0V for a “0” value to around 1.8V for a “1” value.
The input decoder 202 further informs the driver bias generator 204 to turn on to bias the level shifter/drivers 206 and activate the negative voltage generator 208. The negative voltage generator 208 provides the negative power supply voltage to each of the level/shifter drivers 206 to allow the level/shifter driver 206 to control a corresponding FET Q200a, . . . , Q200N (referenced collectively or generally as “FET Q200”). The positive power supply voltage of each level/shifter driver 206 is connected to ground. A high negative voltage from the negative voltage generator 208 allows a high peak voltage as would be found with high power signals. Thus, the level shifter/drivers 206 shifts the output voltage from 0V to 1.8V at the input, to the negative voltage generated by the negative voltage generator 208 to 0V. The voltage presented at the gate of each FET Q200 determines whether the FET is activated, thereby routing the RF signal from the corresponding RF node RF201a, . . . , RF201N to the RF Out node RF200. A negative voltage on the gate turns the FET Q200 off, while a zero voltage turns the FET Q200 on.
In addition, an integrated solution for a switch can generate the negative bias voltage to control a HEMT or other suitable transistor. The negative bias voltage in such integrated solutions is often generated by a charge pump. Charge pumps are generally high impedance due to an available area in an integrated solution. Because of the high impedance of a charge pump or of another source of the negative bias voltage switching times can be long. Using increased, but static, source and driver impedance would allow faster switching, but would increase average current and solution size.